Recent and historic eutrophication of an island lake in northern Lake Michigan, USA

Lake Geneserath is located on Beaver Island in northern Lake Michigan, USA. It exhibits evidence of recent eutrophication, despite limited anthropogenic activity in the watershed. The primary goal of this study was to determine the time scale of trophic state change in Lake Geneserath and the influence of multiple stressors. Sediment cores from the shallow northern and deep southern basins of Lake Geneserath were collected and sectioned at 0.5-cm intervals over the topmost 10 cm and at 1.0-cm intervals for the remainder of the core. Total phosphorus, percent organic matter, and chlorophyll derivative concentrations were measured in each section, and core chronology was established using 210 Pb. Diatom assemblages were quantified from evenly distributed sections in each core. Geochemical variables in the southern basin core indicate historic (AD similar to 1450-1650) and modern (1848-present) periods of eutrophication, coincident with human disturbance and climate warming events. Geochemical variables in the northern basin core confirm accelerating eutrophication after 1950. The fossil diatom assemblages in both basins shifted from species favoring low nutrients to more nutrient-tolerant taxa. A change from periphytic to planktonic diatom species, concurrent with reduced percent organic matter in the northern basin core, indicates rising water level during the second half of the twentieth century. Lake Geneserath was subject to multiple stressors and its trophic status increased with recent residential development and climate warming.